Ballet star Misty Copeland appears for Under Armour. (Under Armour/PR NEWSWIRE)

Six years ago, after tag-teaming an ATM, three Under Armour employees paid a golfer $8,000 in cash to wear the company’s clothes during Masters weekend, jumping at a chance to get the Baltimore-based sportswear upstart’s logo on TV.

Now, the company’s endorsers include not just pro football and basketball Most Valuable Players but a small army of athletic mega-stars who have helped the underdog become one of America’s most formidable sportswear empires.

But a new challenge has arisen: After record-setting, history-making accomplishments for Tom Brady, Misty Copeland, Stephen Curry and Jordan Spieth, expectations for Under Armour are higher than ever — and the competition isn’t backing down.

“There’s no sense of slowdown from our perspective. . . . Momentum is contagious, and it builds on momentum, and we want to be right in the middle of that,” said Adam Peake, Under Armour’s executive vice president of global marketing.

The challenge is “prioritizing the priorities, each one on their own merits . . . the ones we really go after,” he said. “It’s a good problem to have, but at the same time it presents a challenge, because we have to make the right decision for the brand.”

Kevin Plank, chief executive officer of Under Armour Inc., speaks during a news conference in New York, in 2014. (Jin Lee/Bloomberg)

The $19 billion giant, which founder Kevin Plank launched from his grandmother’s Georgetown home to sell sweat-wicking compression shirts, recently passed Adidas to become America’s second-best seller of sportswear, behind Nike.

Under Armour’s stock has gone up nearly 50 percent over the past year, to about $88 on Friday, amid strong endorsement signings and skyrocketing sales. Revenue has grown from about $17,000 in 1996 to more than $3 billion last year.

But the company’s biggest gains came from its athletes, who grabbed trophies, headlines and airtime, all while flexing Under Armour’s brand.

This year began with Under Armour endorser Brady leading the New England Patriots to a Super Bowl victory.

The company’s run continued through this weekend, when Spieth, the 21-year-old golfer who won the Masters and U.S. Open, played for his third straight major championship at the British Open, jockeying for the lead with 22-year-old amateur Paul Dunne, another athlete in Under Armour’s ranks.

In between, Copeland became the first African American ballerina to reach the highest rank of the prestigious American Ballet Theatre, drawing attention to her viral ads for Under Armour and giving the company yet another victory lap.

“Bringing on Misty Copeland,” Adrienne R. Lofton, the company’s senior vice president of global brand marketing, said last month, “is the best decision we’ve ever made.”

The company has ridden their accomplishments all the way to the bank, most notably with Curry, the best shooter in pro basketball, who led the Golden State Warriors to the National Basketball Association championship this year.

The company released its first basketball shoe this year, the Curry One line, and already all of them have sold out online, from the shiny-gold All American to the teal-and-purple Father to Son.

In April, founder Plank said Curry would push the company past its “pretty limited expectations” of a $100 million hoops business into “a $1 billion basketball brand.”

It’s a dramatically risky shot to take: Under Armour’s basketball business would need to grow 30 percent a year for the next decade to hit the capital-B billion. And Nike’s Jordan brand remains a titan, controlling 90 percent of America’s basketball shoe market.

But the company has surprised before. Curry’s signing two years ago, when Under Armour ran about 0.3 percent of U.S. basketball retail, was an extraordinary coup: After Curry played in Nike kicks for four years, Nike let him drop at the end of his contract, allowing Under Armour to swoop in right before his stardom truly took off.

The company is investing in its “I Will What I Want” ad blitz with popular female athletes and role models such as World Cup soccer champion Kelley O’Hara, pro surfer Brianna Cope, supermodel Gisele Bundchen and record-
setting ski racer Lindsey Vonn.

But some of its talent acquisitions have been far more subtle. For its president of global sports categories, Under Armour just poached Terdema Ussery, the longtime chief executive of the Dallas Mavericks and former president of Nike Sports Management, where he handled the Big Swoosh’s athlete marketing arm.

Under Armour may be aiming for more than just a turf war with Nike. With its announcement last week of a new line of sports bras, the company is challenging such “athleisure” and active-wear ­giants as Lululemon and Victoria’s Secret.

Under Armour’s “total addressable market is very big,” according to a Morgan Stanley analyst report last week, “and bigger than thought just last year.”

But Matt Powell, an industry analyst for market researcher NPD Group, said the company can’t simply rely on this year’s big wins.

“The pressure is on now: Can they keep the product flowing?” Powell said. “They’ve got to have continually great product. . . . There’s no place to hide here anymore.”